Fretting is a type of wear caused by two contact surfaces undergoing relative motion under load. The relative motion is often not intended and may be caused by vibration or part deflection under load. The ASM Handbook on Fatigue and Fracture defines fretting as “a special wear process that occurs at the contact area between two materials under load and subject to minute relative motion by vibration or some other force.”
One example of this is rocker shafts in internal combustion engines. Rocker shafts are used to mount rockers. Rockers are activated by camshafts and control valve and injector motion. They oscillate on rocker shafts. The rocker shafts are mounted on rocker shaft mounts which are mounted on the cylinder head of an internal combustion engine.
Fretting can be a problem on the contact surfaces of rocker shaft and rocker shaft mounts, because of rocker shaft deflections.
Prior art solutions have been proposed that minimize movement by providing a close fit between the rocker shaft and rocker shaft mount. For example, U.S. Pat. No. 6,230,676B1 describes that the semi-circular recess of a rocker shaft mount should be dimensioned just slightly larger than the rocker shaft to permit assembly but prevent unwanted looseness or play.